Such self-propelled devices, also referred to as pipeline crawlers, are well known per se. They generally comprise reversible propulsion means energizable from without to advance or retract a mobile carrier inside a pipeline whose weld joints are to be tested by means of a normally deactivated radiation emitter aboard the carrier which is trainable upon these joints for panoramically illuminating same. Any flaw in the weld structure will show up on a radiation-sensitive strip surrounding the joint on the outer pipeline surface.
Earlier devices of this character utilized so-called umbilical cords connected outside the pipeline to a source of control signals. U.S. Pat. No. 4,006,359 describes a pipeline crawler responsive to commands transmitted through the pipeline in the form of pulses of radiant energy picked up by sensors aboard the device.